From c3e42d4bfc931c47aa326588a073b14fb2ad2676 Mon Sep 17 00:00:00 2001
From: Ben Pfaff <blp@cs.stanford.edu>
Date: Tue, 21 Sep 2004 02:48:58 +0000
Subject: [PATCH] Comments. Remove pagedir_clear_page(). Add assertions to
 pagedir_set_page().

---
 src/userprog/pagedir.c | 51 ++++++++++++++++++++++++++++--------------
 src/userprog/pagedir.h | 11 ++++-----
 2 files changed, 38 insertions(+), 24 deletions(-)

diff --git a/src/userprog/pagedir.c b/src/userprog/pagedir.c
index 1edd0ff..6c819c7 100644
--- a/src/userprog/pagedir.c
+++ b/src/userprog/pagedir.c
@@ -6,6 +6,10 @@
 #include "threads/mmu.h"
 #include "threads/palloc.h"
 
+/* Creates a new page directory that has mappings for kernel
+   virtual addresses, but none for user virtual addresses.
+   Returns the new page directory, or a null pointer if memory
+   allocation fails. */
 uint32_t *
 pagedir_create (void) 
 {
@@ -14,6 +18,8 @@ pagedir_create (void)
   return pd;
 }
 
+/* Destroys page directory PD, freeing all the pages it
+   references. */
 void
 pagedir_destroy (uint32_t *pd) 
 {
@@ -36,18 +42,23 @@ pagedir_destroy (uint32_t *pd)
   palloc_free (pd);
 }
 
+/* Returns the mapping of user virtual address UADDR in page
+   directory PD into a kernel virtual address.
+   If UADDR is unmapped, behavior varies based on CREATE:
+   if CREATE is true, then a new, zeroed page is created and a
+   pointer into it is returned,
+   otherwise a null pointer is returned. */
 static uint32_t *
-lookup_page (uint32_t *pd, void *upage, bool create)
+lookup_page (uint32_t *pd, void *uaddr, bool create)
 {
-  uint32_t *pt;
-  uint32_t *pde;
+  uint32_t *pt, *pde;
 
   ASSERT (pd != NULL);
-  ASSERT (upage < PHYS_BASE);
+  ASSERT (uaddr < PHYS_BASE);
 
-  /* Check for a page table for UPAGE.
+  /* Check for a page table for UADDR.
      If one is missing, create one if requested. */
-  pde = pd + pd_no (upage);
+  pde = pd + pd_no (uaddr);
   if (*pde == 0) 
     {
       if (create)
@@ -64,16 +75,25 @@ lookup_page (uint32_t *pd, void *upage, bool create)
 
   /* Return the page table entry. */
   pt = pde_get_pt (*pde);
-  return &pt[pt_no (upage)];
+  return &pt[pt_no (uaddr)];
 }
 
+/* Adds a mapping from user virtual address UPAGE to kernel
+   virtual address KPAGE in page directory PD.
+   UPAGE must not already be mapped.
+   If WRITABLE is true, the new page is read/write;
+   otherwise it is read-only.
+   Returns true if successful, false if memory allocation
+   failed. */
 bool
 pagedir_set_page (uint32_t *pd, void *upage, void *kpage,
                   bool writable) 
 {
   uint32_t *pte;
 
+  ASSERT (pg_ofs (upage) == 0);
   ASSERT (pg_ofs (kpage) == 0);
+  ASSERT (lookup_page (pd, upage, false) == NULL);
 
   pte = lookup_page (pd, upage, true);
   if (pte != NULL) 
@@ -85,21 +105,18 @@ pagedir_set_page (uint32_t *pd, void *upage, void *kpage,
     return false;
 }
 
+/* Returns the kernel virtual address that user virtual address
+   UADDR is mapped to in PD, or a null pointer if there is no
+   mapping. */
 void *
-pagedir_get_page (uint32_t *pd, const void *upage) 
+pagedir_get_page (uint32_t *pd, const void *uaddr) 
 {
-  uint32_t *pte = lookup_page (pd, (void *) upage, false);
+  uint32_t *pte = lookup_page (pd, (void *) uaddr, false);
   return pte != NULL && *pte != 0 ? pte_get_page (*pte) : NULL;
 }
 
-void
-pagedir_clear_page (uint32_t *pd, void *upage)
-{
-  uint32_t *pte = lookup_page (pd, upage, false);
-  if (pte != NULL)
-    *pte = 0;
-}
-
+/* Loads page directory PD into the CPU's page directory base
+   register. */
 void
 pagedir_activate (uint32_t *pd) 
 {
diff --git a/src/userprog/pagedir.h b/src/userprog/pagedir.h
index d3e2ce8..6915a30 100644
--- a/src/userprog/pagedir.h
+++ b/src/userprog/pagedir.h
@@ -5,12 +5,9 @@
 #include <stdint.h>
 
 uint32_t *pagedir_create (void);
-void pagedir_destroy (uint32_t *);
-bool pagedir_set_page (uint32_t *pagedir, void *upage, void *kpage,
-                       bool writable);
-void *pagedir_get_page (uint32_t *pagedir, const void *upage);
-void pagedir_clear_page (uint32_t *pagedir, void *upage);
-
-void pagedir_activate (uint32_t *pagedir);
+void pagedir_destroy (uint32_t *pd);
+bool pagedir_set_page (uint32_t *pd, void *upage, void *kpage, bool rw);
+void *pagedir_get_page (uint32_t *pd, const void *upage);
+void pagedir_activate (uint32_t *pd);
 
 #endif /* userprog/pagedir.h */
-- 
2.30.2